Focussed laser beams are used extensively. One important application is that of the point-of-sale recording system wherein coded labels are scanned with a focussed laser beam and wherein there is always considerable uncertainty as to the distance between the focussing lens and the surface over which scanning takes place. Another important application is found in the high-speed, high-resolution laser printing system wherein there is less tolerance of the uncertainty that does exist to the end that this problem is equally severe in practice.